A modular multi-level converter comprises a plurality of converter cells that are connected in series in each branch of the converter. Each converter cell comprises a capacitor and semiconductor switches for connecting the capacitor to the branch or for disconnecting it therefrom.
Due to the series connection of converter cells, a modular multi-level converter enables AC/DC and DC/AC voltage conversion above the usual upper threshold of about 6 kV AC, which usually is the maximum voltage rating for other power converter topologies. The voltage rating of a modular multi-level converter may reach voltages as high as 20 kV. Thus, a modular multi-level converter docs not require a transformer or complex snubber circuits. Moreover, since the converted power may be increased via a voltage increase, the efficiency of a modular multi-level converter may go up to 99%.
Since each phase of a modular multi-level converter (which usually comprises two branches per phase), comprises a plurality of converter cells, output voltage waveforms of multiple levels may be produced. As the converter cell count increases, the quality of the output voltage waveforms also increases, allowing better approximating sinusoidal voltage waveforms. An operation with low output harmonic distortion of the output voltages and currents is a requirement for most industrial applications, as dictated by various international standards, such as the IEEE 519-1992 and the IEC 61000-3-6.
The series-connected converter cells of a modular multi-level converter may be of relatively low voltage rating, e.g. 1 kV and all converter cells may be identical. Thus, inexpensive semiconductor devices of reduced voltage blocking capability such as 1.7-kV IGBTs may be used. Modularity also enables fault tolerance, as redundant converter cells may be added at low cost.
The high voltage capability, operation at low switching frequency, and modular structure of a modular multi-level converter make it ideal for high-power, medium-voltage industrial applications. However, these advantages come with the increased complexity of control requirements that translate in nontrivial control solutions to guarantee the correct operation of the converter.
A modular multi-level converter is usually controlled by a cascade of control loops: an outer power control loop tracks given reference currents for an active and a reactive power by employing a PI-based decoupled current controller. A current controller provides suitable voltage references for a modulator and an inner balancing control loop which uses the redundancy in the converter states balances the capacitor voltages. The converter cells with lowest capacitor voltage may be prioritized for charging while the converter cells with the highest voltage levels are chosen for discharging currents.
The controller of a modular multi-level converter is usually located physically with the converter and hard-wired with sensors and actuators, increasing the weight, and the space necessary to accommodate the boards of the converter and limiting the computational capability of the controller.
The patent application EP 2515453 A1 discloses a communication system for a power electronic converter made of a plurality of possibly identical converter modules arranged in a converter cabinet and comprising power semiconductor switches or valves. A backbone network carries communication signals between a single, main, central, or higher level converter controller and a plurality of local, or lower level module controllers arranged and/or mounted in the different converter cabinets. The backbone network includes optical fibre which can support high-data rates and are capable of withstanding large voltages.